Studies were undertaken to study the maturation and immunoregulation of in vitro antigen-specific cytotoxic T-cell responses and humoral antibody responses by B-cells in normal individuals and patients with immune deficiency states. Patients were heterogenous with regard to their ability to generate influenza virus specific cytotoxic T-cells in vitro. Most patients with hypogammaglobulinemia produced cytotoxic T-cells normally, while patients with the Wiskott-Aldrich syndrome and ataxia-telangiectasia produced alomost no virus specific cytotoxic T-cells. The latter two patient groups were also deficient in their ability to generate alloimmune cytotoxic T-cells in vitro. Normals produce specific antibody which are macrophage and T-cell dependent. Co-cultures of T-cells with allogeneic B-cells and macrophages with antigen demonstrated allogeneic T-cell help and radiosensitive T-cell suppression. Cord blood cells: 1) produced no specific antibody, 2) had B cells which made no antibody in the presence of mature T-helper cells, and 3) had T-cells capable of providing allogeneic T-helper effects. Normal cells produced mostly IgG antibody and small amounts of IgM and IgA, due to a variation in precursor isotype frequency. No evidence of "isotype switching" was found. Among immunodeficient individuals, cells from 5 of 11 patients with hypogammaglobulinemia who made no antibody in vivo produced specific antibody in vitro. Cells from patients with the Wiskott-Aldrich syndrome and ataxia-telangiectasia produced less antibody than controls. Among the latter patients, defects in both T-cells and B-cells but not monocytes contributed to the poor response. Defects in the production of cytotoxic T-cells and specific antibodies may contribute to the increased incidence of neoplasia observed in immunodeficient patients.